If the heading indicator is inoperative, what is the primary instrument for determining bank?

The primary instrument for determining bank when the heading indicator is inoperative is the attitude indicator. The attitude indicator provides essential information regarding the aircraft's orientation relative to the horizon. It displays whether the wings are level, climbing, descending, or in a coordinated or uncoordinated turn.

Using the attitude indicator, pilots can maintain the desired bank angle as they fly. While the compass can provide directional information, it does not give direct indications of bank or rate of turn in the same way that the attitude indicator does. The turn coordinator is useful for assessing the rate of turn and can indicate the need for a certain degree of bank to maintain a standard rate of turn, but it does not primarily define the aircraft's attitude in space. The vertical speed indicator measures the rate of climb or descent and does not indicate bank. Thus, the attitude indicator is the best choice for determining bank when the heading indicator is not available.